Sec. 2.6 Implementation Of A Nand Logic Gate Using Transistors 17
The diagram in Figure 2.5 uses a common convention: two lines that cross do not indicate an electrical connection unless a solid dot appears. The idea is similar to the way vertices and edges are drawn in a graph: two edges that cross do not indicate a ver- tex is present unless a dot (or circle) is drawn. In a circuit diagram, two lines that cross without a dot correspond to a situation in which there is no physical connection; we can imagine that the wires are positioned so an air gap exists between them (i.e., the wires do not touch). To help indicate that there is no connection, the lines are drawn with a slight space around the crossing point. Now that we have seen an example of how a gate can be created out of transistors, we do not need to consider individual transistors again. Throughout the rest of the chapter, we will discuss gates without referring to their internal mechanisms. Later chapters discuss larger, more complex mechanisms that are composed of gates.
2.7 Symbols Used For Logic Gates
When they design circuits, engineers think about interconnecting logic gates rather than interconnecting transistors. Each gate is represented by a symbol, and engineers draw diagrams that show the interconnections among gates. Figure 2.6 shows the sym- bols for nand, nor, inverter, and, or, and xor gates. The figure follows standard termi- nology by using the term inverter for a gate that performs the Boolean not operation.
Figure 2.6 The symbols for commonly used gates. Inputs for each gate are shown on the left, and the output of the gate is shown on the right.
2.8 Example Interconnection Of Gates
The electronic parts that implement gates are classified as Transistor-Transistor Logic (TTL) because the output transistors in each gate are designed to connect directly to input transistors in other gates. In fact, an output can connect to several inputs². For example, suppose a circuit is needed in which the output is true if a disk is spinning and the user presses a power-down button. Logically, the output is a Boolean and of two 33333333333333333333333333333333 ²The technology limits the number of inputs that can be connected to a single output; we use the term fanout to specify the number of inputs that an output supplies.